The present invention relates to a polyether polyol, a water-curable polyurethane prepolymer obtainable by reacting said polyether polyol with a polyisocyanate compound, and a composition containing such a prepolymer.
Among the hitherto-available polyether polyols for the production of hydrophilic polyurethane resin are polyols which are obtainable by either serial addition or random addition polymerization of propylene oxide and ethylene oxide. For example, Japanese Patent Publication No. 54-15080 describes a process for producing a hydrophilic polyurethane resin using a polyether polyol obtainable by random addition polymerization of ethylene oxide and propylene oxide.
Among the known water-curable polyurethane prepolymers are the prepolymer obtainable by reacting polyethylene glycol with an organic isocyanate as disclosed in Japanese Patent Publication No. 48-25205 and the prepolymer obtainable by reacting a polyol synthesized by random addition polymerization of an alkylene oxide mixture containing not less than 50 weight % of ethylene oxide or a polyol synthesized by serial addition polymerization of propylene oxide and ethylene oxide with an organic isocyanate as described in Japanese Patent Publication No. 53-19846.
However, the .prepolymer synthesized using polyethylene glycol is a solid due to the crystallinity of polyethylene glycol. To make it into a liquid which can be easily used, a solvent or the like must be added but this procedure presents a problem in regard to labor hygiene and a risk of ignition.
The same problem and risk exist with the polyether polyol synthesized by serial addition of propylene oxide and ethylene oxide, for this polyol, too, contains the polyethylene glycol chain.
The polyol synthesized by random addition polymerization of ethylene oxide and propylene oxide is relatively low in viscosity and low in crystallinity, thus being less liable to give a solid. However, since most of its terminal OH groups are secondary hydroxy groups, this polyol is low in reactivity and this not only means a prolonged urethanation time required but, when the polyol is reacted with tolylene diisocyanate (hereinafter referred to as TDI) to prepare a prepolymer, it tends to leave a large amount of TDI unreacted, thus detracting from labor hygiene.